The present invention relates to an X-ray computed tomography (CT) system that constructs tomographic images of a subject through X-irradiation.
In X-ray CT systems, an X-ray generation source that generates X-rays rotates about a subject (patient), and an X-ray detector detects X-rays transmitted by the patient at different angles of projection (this sequence is generally called a scan). The results of the scan are computerized in an operator console, whereby an image (X-ray tomographic image) of a section of the patient containing a region to be imaged (a plane at a sliced position, that is, a slice plane) is produced (reconstructed).
Normally, for the scan, an operator plans to determine the conditions for a scan or the conditions for reconstruction according to a region to be imaged or a purpose of diagnosis. The planning is called scan planning. The X-ray CT system generally includes a user interface helpful for scan planning.
When determining the conditions for a scan, an operator must take account of a patient exposure. The patient exposure can be calculated based on the settings of the conditions for a scan (for example, a tube current, a tube voltage, and a scan time). (An estimated patient exposure to be calculated based on the settings of the conditions for a scan shall be called a CTDI value.) Normally, the X-ray CT system has a facility for indicating the CTDI value through the user interface. The operator can therefore plan a scan at the sight of the CTDI value.
Moreover, some X-ray CT systems have a facility for retrieving the conditions for a scan determined in the past for a patient to be scanned (the conditions for a scan determined in consideration of an exposure in the past) from a database (refer to, for example, Patent Document 1). Using the facility, an operator can readily determine the conditions for a scan permitting high image quality while suppressing a patient exposure.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 9(1997)-299360
However, the conditions for a scan to be determined during scan planning are determined in consideration of an exposure a patient receives during a scan. No consideration is taken into a cumulative exposure representing the sum total of exposures the patient has received in the past during scans. However, if a patient does not recover for a long period of time, the patient may have the same region thereof radiographed a plurality of times during a certain period of time. In this case, even if a patient exposure per radiography is set to a value equal to or smaller than a reference value, the cumulative exposure the patient has received for the certain period of time may exceed the reference value.
In particular, a reference value permissible for a certain a patient's region to be imaged is small (for example, an exposure the eyeballs of the head receive during a certain period of time must be made lower than the exposures of the other regions). When the region including such a part is radiographed, a scan must be planned in consideration of an exposure per scan and a cumulative exposure as well. However, it is hard for the existing X-ray CT systems to plan a scan in consideration of the exposure per scan and cumulative exposure.